ICE on nightly when return type for trait method doesn't match return type for implementation

[jdm]

trait T {
    fn something(&self) -> i32;
}

struct S;
impl T for S {
    fn something(&self) -> i64 {
        panic!()
    }
}

fn main() {
}

yields

$ rustc /tmp/foo.rs
error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: run with `RUST_BACKTRACE=1` for a backtrace

thread 'rustc' panicked at 'called `Option::unwrap()` on a `None` value', ../src/libcore/option.rs:326
stack backtrace:
   1:        0x10e5e24aa - std::sys::backtrace::tracing::imp::write::h482d45d91246faa2
   2:        0x10e5f2260 - std::panicking::default_hook::_{{closure}}::h89158f66286b674e
   3:        0x10e5f057f - std::panicking::default_hook::h9e30d428ee3b0c43
   4:        0x10e5f0c86 - std::panicking::rust_panic_with_hook::h2224f33fb7bf2f4c
   5:        0x10e5f0b24 - std::panicking::begin_panic::hcb11a4dc6d779ae5
   6:        0x10e5f0a32 - std::panicking::begin_panic_fmt::h310416c62f3935b3
   7:        0x10e5f0997 - rust_begin_unwind
   8:        0x10e650c60 - core::panicking::panic_fmt::hc5789f4e80194729
   9:        0x10e650b64 - core::panicking::panic::h1953378f4b37b561
  10:        0x109c3809c - rustc_typeck::check::compare_method::compare_impl_method::_{{closure}}::h08bef90d287aed42
  11:        0x109b968f9 - rustc_typeck::check::compare_method::compare_impl_method::he3538453c2efe177
  12:        0x109bab72e - rustc_typeck::check::check_impl_items_against_trait::hb105ca8bb42a0b3d
  13:        0x109ba8d9b - rustc_typeck::check::check_item_type::hba3733559d49be1e
  14:        0x109ba156e - _<rustc_typeck..check..CheckItemTypesVisitor<'a, 'tcx> as rustc..hir..intravisit..Visitor<'tcx>>::visit_item::h369cacffbaa18923
  15:        0x109ba348c - rustc_typeck::check::check_item_types::h81e44a5d40f450fd
  16:        0x109c18ea8 - rustc_typeck::check_crate::h8ec8b2f490f5dc28
  17:        0x109917d8a - rustc_driver::driver::phase_3_run_analysis_passes::_{{closure}}::h45e03cee16dcf298
  18:        0x1098e578b - rustc_driver::driver::phase_3_run_analysis_passes::hc0de40cea97d81a1
  19:        0x1098d3231 - rustc_driver::driver::compile_input::hd9ecc57abd3cba85
  20:        0x109904239 - rustc_driver::run_compiler::h184264500271cc39
  21:        0x109844b74 - std::panicking::try::do_call::h17a7a17ad7240c5c
  22:        0x10e5f8f9a - __rust_maybe_catch_panic
  23:        0x10985ec67 - _<F as alloc..boxed..FnBox<A>>::call_box::h93f9128277b2964a
  24:        0x10e5ef345 - std::sys::thread::Thread::new::thread_start::he0bf102845911132
  25:     0x7fff945f2898 - _pthread_body
  26:     0x7fff945f2729 - _pthread_start

This exists at least as far back as the 8/22 nightly.

[TimNN]

Probably a duplicate of #35869.

[jdm]

Yep.